This invention relates to the pyrolysis of kerogen-containing oil shale and is particularly concerned with an oil shale retorting process in which the retorted shale is burned to produce hot, decarbonized solids that are used to supply heat for retorting shale fines and for heating recycle retort gas.
Many methods for recovering oil from kerogen-containing oil shale have been proposed in the past. The majority of these methods involve pyrolysis which is commonly referred to as retorting. To be competitive with the production of oils from petroleum stocks, it is essential to recover as much of the heat value from the organic material in the oil shale without incurring prohibitive expense or environmental damage. Normally, oil shale contains between about twenty and about eighty gallons of oil per ton and only a limited proportion can be recovered as product oil or gas. Economical retorting must utilize the remaining heat energy contained in the shale to provide heat for pyrolysis. Sulfur emissions in the gases released from the retorting process, however, must be restricted to low levels required by law while attempting to utilize more of the organic material in the shale.
It is known to retort oil shale by a technique of contacting upward flowing, hydrocarbon-containing solids with downflowing gases in a vertical retort. One such technique is disclosed in U.S. Pat. No. 3,361,644, the disclosure of which is hereby incorporated by reference in its entirety. To produce product vapors, the upward-moving bed of shale particles exchanges heat with a downflowing, hydrocarbonaceous and oxygen-free eduction or retorting gas of high specific heat introduced into the top of the retort at a temperature between about 950.degree. F. and about 1200.degree. F. In the upper portion of the retort, the hot eduction gas pyrolyzes the shale, thereby producing hydrogen and hydrocarbonaceous vapors. In the lower portion of the retort, the eduction gas preheats the ascending bed of shale particles to retorting temperatures. As preheating continues, the eduction gas steadily decreases in temperature, condensing high boiling hydrocarbonaceous vapors into a raw shale oil product while leaving a product gas of relatively high BTU content. The shale oil and product gas are then separated, and a portion of the product gas is heated and recycled to the top of the retort as the eduction or retorting gas.
To minimize the volume of recycled gas required, upflow retorting is usually conducted at pressures above atmospheric with the pressure in the upper regions of the retort normally ranging between about 5 p.s.i.a. and about 100 p.s.i.a., preferably between about 25 p.s.i.a. and about 65 p.s.i.a. The operation of the retort at superatmospheric pressure, however, means that provisions must be made for introducing and recovering particulate shale from the retorting zone without allowing valuable product and recycle gases to depressure. Conventional methods for achieving these objectives use elaborate lock vessels, valves, or slide valves which tend to wear rapidly and produce excessive fines by abrading the shale. Alternatively, liquid sealing devices, such as described in U.S. Pat. No. 4,004,982, have been employed. These devices operate by moving shale particles through a standing head of oil or water, thereby creating a positive back pressure to prevent escape of retort gases. Liquid seals effectively contain retort gases but leave the shale wet. When incorporated into a process for combusting retorted shale in a vessel separate from the retort, use of liquid seals would require the expense of drying the shale prior to combustion.
To increase product yield beyond that which can be educed in the retort alone, processes have been developed to generate product gases by reaction of hot, retorted shale with an oxidizing gas stream, for example, as taught in U.S. Pat. No. 4,010,092, the disclosure of which is hereby incorporated by reference in its entirety. Such gasification reactions conducted in an oxidizing environment, however, burn the coke on the retorted shale at temperatures high enough to release significant amounts of carbon dioxide from decomposing carbonates in the shale. This, in turn, necessitates expensive removal of carbon dioxide from the combustible product gases. Another source of product yield is unretorted shale fines. Oil shale mined for the purpose of retorting in above-ground retorts is usually crushed mechanically to a size suitable for retorting, normally a top size of about 3 inches or smaller. Due to the friable nature of shale, fines ranging in size up to about 1/8 inch are generated in the mining and crushing of larger particles in amounts up to about 10 weight percent of the total shale mined. In above-ground retorting processes, fines mixed with the feed of larger, retort-size particles tend to fill the void spaces between the larger particles. As a result, circulation of hot eduction gases is restricted thus reducing the retort throughput and its oil producing capacity. When the fines are segregated from the feed to the retort to avoid this problem, an appreciable portion of energy available from the shale is wasted.
Retorted shale contains heat energy in the form of coke which can be recovered by passing the retorted shale particles through a combustion zone to burn the coke. Retorted shale, however, generally contains sulfur components and less than complete combustion of the coke generates hydrogen sulfide, which must be removed from the flue gases by means of costly sulfur recovery processes. On the other hand, complete combustion may result in flue gases containing unacceptable quantities of sulfur dioxide. U.S. Pat. No. 4,069,132 discloses a combustion process wherein sulfur dioxide generated during the complete combustion of coke on retorted shale is converted to stable inorganic salts by reaction with alkaline ingredients in the shale. This process utilizes a combustor through which hot retorted shale passes co-currently with air diluted by sufficient flue gas to control peak combustion temperature below about 1670.degree. F. Under such conditions, the discharge of sulfur dioxide from the combustor is disclosed to be greatly reduced.
Because flue gases from combustion zones associated with shale retorts are usually at high temperatures, many retorting processes are designed to utilize the heat contained therein. One example is taught in U.S. Pat. No. 4,069,132, which discloses a process in which hot flue gases are passed in indirect heat exchange with boiler feed water to generate process steam.
Even though many of the above-discussed features have been incorporated into oil shale retorting processes, the need still exists for further developments to improve the efficiency of the processes by effectively retorting raw shale fines using heat generated in the process while controlling the emissions of hydrogen sulfide and other undesirable gases into the atmosphere.
Accordingly, it is one of the objects of the present invention to provide a process for recovering oil and gas from raw shale fines by retorting the fines utilizing the heat contained in decarbonized solids produced by combusting pyrolyzed shale particles. It is another object of the invention to provide a process to heat the recycle retort or eduction gas used for pyrolysis in the retort utilizing the heat contained in decarbonized solids produced by combusting pyrolyzed shale particles. It is yet another object of the invention to utilize decarbonized solids produced by combusting retorted shale particles to remove a large proportion of hydrogen sulfide and carbon dioxide from the gases produced during retorting in order to minimize processing requirements for product gas clean up. These and other objects of the invention will become more apparent in view of the following description of the invention.